A steelmaking converter is provided with a taphole for discharging molten steel to a ladle or the like therethrough, wherein the taphole is defined by a tubular-shaped refractory member. Generally, this refractory member is called, “taphole sleeve brick” or simply “sleeve brick”.
The taphole sleeve brick is required to have thermal shock resistance and oxidation resistance, because it is exposed to rapid changes in temperature and atmosphere during a waiting or shutdown period after a tapping operation in a converter, and further required to have abrasion (erosion) resistance and high strength, because it is severely exposed to a high-temperature molten steel stream during the tapping operation.
An unburned magnesia-carbon based material is widely used as a refractory material for the taphole sleeve brick. Although the magnesia-carbon based material is excellent in thermal shock resistance and therefore suitable for the taphole sleeve brick to be used under severe thermal shock conditions, there remains a strong need for further extending a durable period of the taphole sleeve brick to improve a converter-operating rate.
Heretofore, in order to enhance oxidation resistance and strength so as to achieve longer durable period, a metal additive, such as aluminum or boride, has been used as effective means therefor. The addition of a metal powder, such as an aluminum powder, has a significant strength-enhancing effect which arises from creation of secondary bonds based on carbide and spinel formation and from matrix densification based on volume expansion, and simultaneously enhances abrasion resistance. On the other hand, it causes significant deterioration in thermal shock resistance. Thus, the metal powder is generally added in a small amount.
It has also been practiced to reduce a content of carbon, such as graphite, i.e., use a low-carbon material, to obtain a strong matrix based on matrix densification and formation of an MgO-rich layer on an operating surface, so as to suppress abrasion (erosion) due to a molten steel stream, and oxidation. Typically, a magnesia-carbon brick contains flake graphite in an amount of about 20 mass %. It is known that, if the flake graphite is drastically reduced to a level of 10 mass % or less, abrasion resistance and oxidation resistance will be enhanced, which leads to improvement in durability.
For example, the following Patent Document 1 discloses a low-carbon MgO—C refractory material for a converter taphole, which contains 1 to 8 weight % of a carbon raw material, 0.3 to 5 weight % of pitch, 0 to 1.5 weight % of a metal additive and 0 to 0.8 weight % of boride. It is described that this refractory material is characterized by containing a relatively small amount (1 to 8 mass %) of carbon, and a given amount of pitch powder, which makes it possible to obtain an advantage of being able to suppress oxidation damage without spoiling excellent thermal shock resistance of the MgO-based refractory material, and have high post-burning bending strength, and excellent slag-corrosion resistance.
The following Patent Document 2 discloses a converter sleeve brick produced by adding 3 to 5 mass % of organic binder, tar or pitch, to a refractory material which comprises 60 to 90 mass % of magnesia, 5 to 35 mass % of graphite, 1 to 10 mass % of aluminum powder, and 0.1 to 20 mass % of chromium-alloy fibers, and subjecting the resulting mixture to kneading and forming. It is also disclosed that oxidation resistance and thermal shock resistance in the obtained converter-taphole sleeve brick are enhanced by adding/mixing the chromium-alloy steel fibers thereto.
The following Patent Document 3 discloses a taphole sleeve for steelmaking converters, excellent in slag resistance, which is prepared by subjecting a mix containing resin-coated magnesia particles and carbon, to a forming process.    [Patent Document 1] JP 8-259312A    [Patent Document 2] JP 6-220517A    [Patent Document 3] JP 2000-309818A